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UCD3138: Questions of pin impedance of UCD3138

HAN LEE
Expert 1515 points
Part Number: UCD3138

Hi Experts,

We know some pins of UCD3138 are multi-function ,like DPWM.UART which was able to set as gpio , but we don't know what's the pin impedance of these pins when it has been set to in/out gpio?

I was wondering if the direction of gpio  would affect the result of impedance measured or not? 

Other question ,are there ESD diodes at each pins, right? does it would affect the pin impedance that i measured for good/ng IC judgment? thanks.

  • 0
    TI__Expert 4070 points

    Hello,

    An engineer is looking into this and will respond.

  • 0 in reply to Amiel Hussain
    TI__Genius 17090 points

    We don't make any specification for impedance with voltages outside the rails more than .3 volts.  So it depends on what voltage and polarity you are applying with your voltmeter.  If the pin is configured as an input, it will generally have a very high impedance.  Several tens of megohms.  If it's configured as an output, it's going to have a much lower impedance. It will be more than capable of driving the 4 milliamps or so speced in the data sheet.  That spec comes from limitations on internal power buses, temperature and so on, not on impedance.  

    There are ESD diodes on most of the pins, except some of the PMBus pins are set up without a diode on the top side, so that you can have buses running into a UCD which is not turned on.  However the ESD diodes are not intended to be used for things like running 5 volt signals into a device with a 3.3 volts supply.  It's probably not going to work well if you try that.  You need external protection diodes for something like that.  

    Note that the EADC pins and ADC pins have lower impedances.  The EADC has a buffer, and its impedance is in the data sheet.  The ADC has leakage and a capacitor that needs to be charged everytime you sample.  So it's imput impedance is a function of the sample rate.  The more often you sample, the lower the impedance.  We recommend that you treat is as having a 1K ohm input impedance.  

  • 0 in reply to Ian Bower
    Expert 1515 points

    Hi Ian,

    Thanks for  your replies!

    Would you recommend with/without supply voltage to IC  when we measured the pin impedance for NG IC judgment?

    What's the reasons?

    Actually ,we have a IC that seems to be broken,we think the failure symptom  is on logic gate for those which was able to set multi-function pins,

    and we cannot observe any abnormal at that pins by measured pin impedance,can you give us some suggestions for this case? We would like to do FA 

    For ti failure analysis rules, this IC cannot be accepted if we have nothing abnormal data,right?  

  • 0 in reply to HAN LEE
    TI__Genius 17090 points

    You definitely want to have the device powered up to measure impedance. 

    What failure are you seeing? If you can let me know, I can maybe help you debug it, and maybe tell you whether you should return it.  

    If you suspect a pin failure, and you can go to ROM mode, just verify that the pin can pull high and low by writing to I/O regsters with the memory debugger in ROM mode.  If it's an I/O, see if you can read the correct value in the same way.  Rather than using a voltmeter, which will put an unknown voltage in an unknown direction, put a resistor in there, and see if it can pull the pin up or down.  Typical scope impedance is 10 megohm, that should pull the pin down no problem if it's an input.  So if you're using a scope, maybe try a 1 megohm to pull it up?  For an output, it should be able to pull a 1K ohm pretty far up or down.  

  • 0 in reply to Ian Bower
    Expert 1515 points

    Hi Ian,

    Actually , We have a  case  that  pin17 was set gpio input for fan control ;PSU was shut down casued by pin17 detected offset 1V, and this issue was solved after we replaced a brand new UCD3138, the pin impedance of this IC is same as other ICs,and I have wrote some code  to test pull high/down capability with this pins, it's workable (I set DPWM0B as gpio input, DPWM0A as gpio output , if DPWM0B detected high, DPWM0A would light LED on open socket board ) 

    what else can I do for this case,thanks.

  • 0 in reply to HAN LEE
    TI__Genius 17090 points

    I guess I missed this return message.  Since it's been so long, I'm going to close it.  If you still have a question, please open another question